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Physical review. B, Condensed matter

Time-resolved study of dephasing mechanisms of excitons in GaAs/ Alx Ga1−x As quantum-well structures

Huib J. Bakker, Karl Leo, Jagdeep Shah, Klaus Köhler

Published Mar 15, 1994• 20 citations

Abstract

We present an experimental and theoretical study of the dephasing of heavy-hole excitons in a 70-\AA{} GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-well stucture. We measure the time-integrated three-pulse four-wave-mixing signal of heavy-hole excitons in the quantum-well sample. We model the third-order nonlinear optical response incorporating the effects of a stochastic modulation of the transition frequency. We find that the dephasing of the nonlinear polarization is mainly the result of a spectral-migration process of the excitons and, to a lesser extent, due to a homogeneous dephasing process. The time dependence of the four-wave-mixing signal is very well described by theory if the spectral-migration process is assumed to be energy dependent.

DOI: 10.1103/physrevb.49.8249

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We present an experimental and theoretical study of the dephasing of heavy-hole excitons in a 70-\AA{} GaAs/

{\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}

As multiple-quantum-well stucture. We measure the time-integrated three-pulse four-wave-mixing signal of heavy-hole excitons in the quantum-well sample. We model the third-order nonlinear optical response incorporating the effects of a stochastic modulation of the transition frequency. We find that the dephasing of the nonlinear polarization is mainly the result of a spectral-migration process of the excitons and, to a lesser extent, due to a homogeneous dephasing process. The time dependence of the four-wave-mixing signal is very well described by theory if the spectral-migration process is assumed to be energy dependent.

We report experiments on degenerate four-wave mixing in a high-quality GaAs/

{\mathrm{Al}}_{0.3}

{\mathrm{Ga}}_{0.7}

As quantum well using the time-resolved self-diffraction technique. Frequency-dependent measurements around the heavy-hole exciton transition reveal a strong biexcitonic nonlinearity.

We investigate the spectral properties of the excitonic nonlinear response in a GaAs quantum-well sample containing wells of different thicknesses. We observe both the heavy-hole biexciton and the light-hole biexciton in the transient four-wave mixing spectrum and estimate their binding energy dependence on well width. For moderate excitation densities the signal for negative delay at the heavy-hole exciton and biexciton resonances is due to two-photon transitions to the biexciton states through the exciton states. The temporal behavior of the transient four-wave mixing signal from the heavy-hole exciton resonance is strongly dependent on the spectral position of the laser. We show that this effect is due to interference phenomena and to excitation-induced dephasing.

Shifts in the relative phases of quantum beats in the time-resolved four-wave-mixing signals from a GaAs/

{\mathrm{Al}}_{\mathit{x}}

${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum well are observed. The relative phases are shown to depend strongly on the intensities of the incident pulses as well as their detuning from the heavy-hole exciton. A simple coupled six-level model shows that the phases of the quantum beats are directly determined by many-body effects such as density-dependent dephasing and the local-field correction.

Three-beam time-integrated transient four-wave mixing (TFWM) is used to probe both light-hole (LH) and heavy-hole (HH) exciton coherences in weakly disordered GaAs quantum wells. The period of the quantum beats between the LH and the HH exciton coherences changes as a function of first-second pulse delay. A theoretical model for the TFWM signal, including exciton dephasing rates that vary with frequency across the inhomogeneous linewidth, reproduces the observations. The inclusion of inhomogeneous dephasing also improves the correspondence between theory and experiment when data as a function of both first-second and second-third pulse delays are simultaneously examined.

We report time-resolved degenerate four-wave-mixing experiments on a high-quality single GaAs quantum well. The signals for parallel and cross-polarized exciting pulses show pronounced quantum beats with different frequencies corresponding to the heavy/light-hole splitting and the biexciton binding energy, respectively. Comparison of the experimental data with solutions of the optical Bloch equations for a phenomenological model system shows that these features result from strong contributions of the biexcitonic state. The importance of biexcitons to the nonlinear coherent response of the two-dimension exciton is further confirmed by the frequency dependence of the time-integrated four-wave-mixing signal amplitudes measured for the two polarization geometries.

We report the experimental results of two-pulse transient four-wave mixing near the first heavy-hole quantum-well exciton. Input pulses with similar polarizations (circular or linear) measure the exciton dephasing rate; opposite circular polarized pulses produce no four-wave mixing signal, while crossed linear polarized pulses generate a weaker signal with a faster dephasing rate. This signal is attributed to the biexcitonic transition, which was directly observed in a separate nondegenerate four-wave mixing experiment. The selection rules for these transitions are discussed and confirm the treatment of the excitonic transition as a three-level system.